The invention relates to a signal track circuit where the indication of the presence of a rail vehicle is obtained by way of signals transmitted from two or more signal sources.
Currently used parallel track circuits indicating the presence of a rail vehicle at a certain track section, operate on the principle of the receipt of a signal at one end of a track section when that signal is introduced at the opposite end of the particular track section. There is also an arrangement wherein the signal transmitter is at the center of the track section while a receiver is located on each end of the track section, both receivers cooperating when determining whether the section is free or occupied. Track circuits with limiting insulated rail joints are predominantly used where, on electrified tracks and at lengths over 1.5 km, a shunt sensitivity of about 0.1 ohms is achieved. At present track circuits without insulated rail joints are preferred for their increased reliability in operation. They show, however, a number of drawbacks:
they operate with signal frequencies within a range above 1.0 kHz, making them more sensitive to influences of foreign currents, particularly from traction vehicles, PA1 they are only effective over a technical length generally shorter than 1.0 km with increased demands on the insulating conditions of the track circuit (insulating conductivity generally lower than 0.5 S/km), PA1 a low level of shunt sensitivity (0.06 ohm) is exhibited, and PA1 the accuracy of determination of the operating length is generally worse than 10 m.